1. Field of the Invention
The invention relates to video teleconferencing and, more particularly, to apparatus for an interactive graphics transmission system.
2. Description of the Prior Art
Video teleconferencing is often used where several individuals scattered throughout various remote geographic locations desire to confer together without the necessity to travel to a common locale. With video teleconferencing, each conferee or user is continuously photographed by a video camera in order to be seen by the others, and each conferee sees the others through various video monitors.
During such conferences, graphical information is often discussed and shown, or exhibited, by one or many of the conferees in order to be viewed by the others. Separate video cameras are used to photograph images of the conferees and the graphic being shown. In so-called "continuous presence" video teleconferencing, any conferee must always be able to view the other conferees as well as simultaneously see any graphical information under discussion. A typical transmission over a single communication channel thus contains both a "full-motion" video image of the conferees and graphical information such as, for example, printed pages, charts, or the like. Since the graphic is presumed not to change while it is being discussed, it is typically transmitted with high resolution in a "snapshot" or single-frame mode to reduce its transmission time. At a receiving site, the graphic is usually displayed on a separate monitor.
Oftentimes, it is necessary for a conferee to modify the graphic. A light pen or stylus, operating in conjunction with the graphics transmission system, is frequently used by the conferee to define a desired graphical modification. These modifications could involve the conferee pointing to, i.e., emphasizing, and/or changing, i.e., writing or erasing, selected portions of the graphic. Whenever presently available video teleconferencing systems are used to transmit such a modification, these systems usually transmit the entire graphic including the changed portion. Unfortunately, whenever the graphic is transmitted, the transmitted video image of the conferees is either disadvantageously degraded in proportion to the fraction of the channel bandwidth used to transmit the graphic or, more likely, completely preempted by the transmission of the graphic. Consequently, during these times, each conferee will only see an image of the graphic and will not see an image of the other conferees.
It has been recognized that because the amount of information actually needed to modify the graphic is quite small, a substantial amount of transmission bandwidth is wasted by retransmitting the entire graphic.
One solution which markedly reduces transmission bandwidth is to use a scheme based upon conditional replenishment, i.e., one which transmits an initial video image of the entire graphic followed by the changes occurring in that graphic. See, for example, that shown in U.S. Pat. No. 3,553,362, issued Jan. 5, 1971 to F. W. Mounts. Unfortunately, a conditional replenishment based system transmits all the information contained within the field of view of the video camera which is photographing the graphic. Consequently, much more information is transmitted than just the changes occurring in the graphic. This other information may, for example, be comprised of an image of the conferee, or user, or a portion, e.g., a hand, of the conferee who is changing the graphic and is within the field of view of the camera. Information of this sort is extraneous and, when transmitted, highly wasteful of transmission bandwidth.
It is known that in order to achieve maximum bandwidth reduction, using an existing conditional replenishment based graphical transmission system, the conferee could first change the graphic and then signal the transmission equipment, by, for example, depressing a pushbutton, that the changes are ready to be transmitted. If the graphic is frequently changed, this latter requirement is quite burdensome to the conferee. Moreover, since it necessitates extra effort on the conferee's part, this requirement tends to limit the rate at which the conferee can manually supply changing graphical information to the transmission system.